rotaxanes
(rus. ротаксаны)
— molecules consisting of a ring and an open chain threaded through the ring. Spatial groups at the ends of the chain do not let the ring and the chain disengage.

Description

The ring and the linear chain in rotaxanes are not chemically bound to each other and are held together by steric hindrance created by bulky end groups. Sometimes this is referred to as "mechanically bound". When a linear chain coils into a ring, catenanes are formed, i.e. molecules that have several rings threaded into each other. Rotaxanes may be composed of several rings.

Under certain conditions, a ring can reversibly migrate along the chain (see the Figure); this means that such rotaxanes can serve as molecular nanomechanical devices.

Illustrations

Rotaxan in which a cycle may move along the chain as a result of pH changes.